WO2004037510A1

WO2004037510A1 - Device and method for producing physically expanded structural foams during an injection molding process involving the use of dynamic mixing elements

Info

Publication number: WO2004037510A1
Application number: PCT/EP2003/011197
Authority: WO
Inventors: Christian Schlummer
Original assignee: Peguform Gmbh
Priority date: 2002-10-22
Filing date: 2003-10-09
Publication date: 2004-05-06
Also published as: ES2359698T3; ATE501824T1; US20060034958A1; EP1575752B1; AU2003273976A1; US7293982B2; EP1575752A1; DE10249314B3

Abstract

Foamed molded articles can be produced during an injection molding process, among other things, by using physical expanding agents. Existing technologies differ in the greatest possible extent as to how the expanding agent is introduced into the polymer melt. One possibility consists of feeding the expanding agent via mixing elements (9), which are distributed over the screw plunger (1), are made of a porous and permeable material, and which, by means of an appropriate supply of expanding agents, load and homogeneously distribute the melt with expanding agent during the metering phase of the polymeric material. When injected into an injection mold, the single-phase polymer/expanding agent mixture produced in the aforementioned manner forms a foam structure as a result of a reduction in pressure.

Description

Device and method for producing physically driven structural foams in the injection molding process using dynamic mixing elements

The invention has for its object to introduce and distribute a physical blowing agent with high reproducibility and process reliability evenly in the melt flow of an injection molding machine in order to generate a homogeneous polymer / blowing agent solution, using a conventional injection molding machine.

From the patents DE 24 02 203 C3 and US 5297 948 A devices for the production of foamed plastic molded parts are known according to the preamble of claim 1 with the restriction to an extrusion process, in which the blowing agent is introduced only in some, locally restricted areas. This invention is a further development of the device claimed in our patent application EP 1 256 430 A1 for producing foamed plastic molded parts. Said device is an injection molding machine which is used to produce a foamed plastic molded part. In order to produce a foamable plastic mass, a blowing agent is added to the plastic material, which produces gas bubbles in the injection mold by expansion of the blowing agent dissolved in the melt under pressure as a result of pressure reduction during injection into the injection mold, which are frozen as a result of an increase in viscosity during cooling of the melt and ultimately form the foam structure. Physical blowing agents are used for the device and the method which are presented in EP 1 256 430 A1. A physical blowing agent is introduced into a polymer melt through a porous sleeve. This porous sleeve is mounted on the screw piston, preferably in an area between the metering zone and a downstream mixing zone. The porous sleeve is made of porous or permeable material through which the physical blowing agent passes under pressure to dissolve in the melt. This porous sleeve is ideal as a thin-walled, cylindrical part for the gas entry for polymer melts of various compositions, since it has a large surface. The solution presented in patent application EP 1 256430 A1 relates to gassing with a subsequent mixing process by means of one on the screw piston mounted mixing element. Fumigation takes place in a section of the screw piston, which means that the gassing element carries out the movements of the screw piston. By using a gassing element in a section of the screw piston, the investment costs of the entire system are reduced, because in a conventional injection molding machine only the screw piston has to be replaced in order to produce molded plastic parts with the same system. The use of a gassing device in the screw piston is known from DE 2053646 B, but the propellant openings designed as injection nozzles open into the distributor head. The blowing agent is introduced into the melt in the form of a jet through the blowing agent openings. The design of the gassing element as a porous sleeve, which moves axially with the screw piston and at the same time also carries out its rotational movements, results in an even introduction of propellant, because no jet can be created through the porous surface, but at most a beam, but generally through the

Device according to the invention or EP 1 2256430 A1 blowing agent bubbles introduced into the polymer melt. In the immediate vicinity of the porous sleeve, the gas entry results in incomplete mixing of the gas with the polymer melt, since shear forces, which facilitate the mixing, are low on the circumference of the smooth sleeve.

On the other hand, good mixing effects can be achieved by shearing, stretching and rearranging the melt.

A possible way to achieve this goal is shown in DE101 50 329 A1. The compressed blowing agent is brought into contact with the melt via a static mixing element, which is installed between the plasticizing unit and the sealing nozzle. A porous sintered metal surface, which surrounds the mixing elements, serves as a contact element between the blowing agent and the polymer melt. Differences in concentration and pressure cause absorption of the blowing agent in the melt via diffusion and sorption processes. The polymer / blowing agent mixture is homogenized during the injection process by the webs of the static mixing element which interrupt the melt channel. The rearrangements, distributions and expansions of the melt within the mixer favor the diffusion processes. The absorption of the blowing agent in the melt is sustainably promoted. A disadvantage in the invention disclosed in DE 101 50 329 A1 is that the propellant is only introduced shortly before the sealing nozzle. So little remains Time for complete mixing of the melt before it passes through the sealing nozzle into the adjoining cavity. In order to ensure complete mixing of the melt with the blowing agent, either a mixing element with a long overall length or a high pressure must be applied to the mixing element so that the blowing agent is evenly distributed in the polymer melt before it enters the cavity via the sealing nozzle , EP 1 256 430 A1 also mentions the basic disadvantage of static mixing elements as the shear effect thereof, which can damage the polymer matrix. Another disadvantage of using a static mixing element in the area of the screw piston is the complex valve control that is used to regulate the

Propellant entry serves, which increases the plant costs and the susceptibility to failure. When constructing the porous sleeve according to EP 1 256 430 A1, there is a risk that leaks will occur during operation due to the large sealing surfaces, as a result of which the blowing agent no longer reaches the polymer melt exclusively through the porous sleeve, but also via the sealing points. In addition, if the pressure drops due to a malfunction in the blowing agent system, the case could arise that the polymer melt, which is under higher pressure, gets into the blowing agent supply system via such leaks. In order to remedy these disadvantages of the prior art, it is proposed according to this invention to use at least one dynamic mixing element, that is to say a mixing element which can be moved with the screw piston and via which the propellant is simultaneously introduced. The invention advantageously provides that the screw piston downstream of a metering zone has porous or permeable mixing elements which can be acted upon by the blowing agent via a blowing agent supply device in the core of the screw piston and introduce the blowing agent evenly into the melt. During the plasticizing phase, the mixing elements rotate in the polymer melt with simultaneous translational movement of the screw piston. This combination of translation and rotation during the metering phase ensures constant mixing and rearrangement of the melt with simultaneous exposure to blowing agent and thus ensures a homogeneous polymer / blowing agent mixture.

The combination of the mixing element and the gassing area in the same section of the screw piston not only allows a combination of the mixing element and propellant entry in a narrowly defined section of the screw piston. The The geometric shape of the mixing element as a rotationally symmetrical body also allows a precise introduction of blowing agent into the polymer melt. Furthermore, the amount of blowing agent input can be precisely controlled. The design of the mixing elements as rotationally symmetrical bodies, which protrude into the melt in the outgassing area, ensures uniform mixing and homogenization of the blowing agent input. Due to the rotation and translation of the screw, the mixing takes place even with a short residence time of the melt in the gasification area. Only the mixing element itself consists of porous or permeable material, the screw piston element can consist of a material that has a higher strength. The dynamic forces acting on the mixing elements, which are caused by the movement of the mixing elements in the melt, thus act only on small mixing elements, preferably designed as rotationally symmetrical bodies. As a result, the loads caused by shear or torsional forces can be reduced to a minimum.

The mixing elements themselves have a seal which ensures that the gas is introduced exclusively through the porous surface. This means that the size of the blowing agent inclusions can be set precisely over the entire melt surface.

With the help of the invention, it is possible to produce physically driven structural foam molded parts with only minor changes to a conventional injection molding machine, which are characterized by a compact outer skin and a foamed core and thus, compared to compact components, material-specific advantages with savings in weight, material and associated with costs. Furthermore, no intervention in the machine control is necessary, so that the investment costs are low.

The invention has the following advantages over the prior art:

• Low investment costs, since no complex special machine is required, just an exchange of the screw piston of a conventional injection molding machine.

• Uniform blowing agent introduction due to several axially moving and rotating gassing points during polymer dosing. • High degree of homogenization due to intensive mixing processes with an effective length of the mixing and shear zones of the screw piston that is always the same in the course of the gassing.

• Optimal solution behavior due to long diffusion times and large diffusion areas with small diffusion paths.

• Reproducibility of the process regardless of the dosing volume.

• High efficiency of the blowing agent.

• Easily interchangeable defective or clogged mixing elements

• Possibility of combining mixing elements of different designs and various optimization options depending on the one to be processed

polymer material

The fact that the blowing agent is introduced evenly into the polymer melt via the porous or permeable mixing mandrels makes it possible to introduce the blowing agent optimally during the metering of the polymer. This results in an improved solution behavior due to long diffusion times and large diffusion areas with small diffusion paths. In addition, a high reproducibility of the injection molding process can be determined regardless of the dosing volume and an optimal use of the blowing agent. The rotational and translational movement of the mixing elements in the polymer melt and the associated shear effect prevent local concentration differences and blowing agent agglomerates. Finally, the invention has the advantage of low investment costs, since no complex special machine is necessary, but only an exchange of the screw piston of the conventional injection molding machine. An extended injection unit is also not necessary. A standard length of the injection unit in the range of 20 to 25 times the outer diameter of the screw piston is sufficient.

It is preferably provided that the diameter of the screw piston is reduced in the area of the porous or permeable mixing elements of the screw piston. Due to the low pressure level of the polymer melt in the gassing area, the increased screw depth enables the blowing agent to be supplied directly without the need for a dosing station. The mixing elements are preferably provided evenly offset in several rows on the circumference of the screw piston in order to ensure a uniform distribution of the propellant fluid in the melt.

The propellant is preferably added to the screw plunger

The high-pressure seal housing, which surrounds the screw piston, is fed in during the metering phase. The physical blowing agent is a fluid.

The high-pressure seal housing receives the propellant from at least one pressure bottle. This has the advantage that no dosing station is required.

The high-pressure seal housing moves simultaneously with the axial movement of the screw piston without rotation in the axial direction. This enables a uniform introduction of propellant due to the flat, axially moving and rotating gassing area during the polymer metering.

The polymer / blowing agent solution is homogenized with an effective length of mixing and shear elements of the visual bulb while the gassing is always the same. The propellant is injected during the dosing phase.

Fig. 1 is a sectional view of an injection molding machine with a screw piston

Fig. 2 shows a detail of a mixing element

Fig. 3 shows a possible arrangement of the mixing elements on the screw piston

The invention is explained in more detail below with reference to the drawing in FIGS. 1 and 2:

1 shows an injection molding machine with a screw piston 1 rotating in the injection unit 2 and axially moved during the injection phase. The polymer granulate is fed via a material hopper 3 and drawn in by the rotating screw piston 1 in the region of a feed zone 4. The subsequent compression zone 5 and metering zone 6, with the aid of the external cylinder heating 7, cause the polymer material to be melted, compressed and homogenized, so that a thermally and materially homogeneous polymer melt is present at the end of the metering zone 6. At the end 8 of the metering zone 6 of the screw piston 1, the screw base is increased 8, i.e. the

The diameter of the screw piston 1 is reduced suddenly. In the area of Reduced diameter porous or permeable mixing elements 9 are provided, which can be acted upon by a blowing agent supply device 10 and a bore 11 with a physical blowing agent, the blowing agent being introduced evenly into the polymer melt. The porous or permeable mixing elements 9 serve as a contact surface between the blowing agent and the polymer melt. The change in the basic depth of the screw piston leads to a reduction in pressure in this section, the so-called gassing zone 13. The compressed propellant, for example a propellant fluid, is supplied via the bore 11 in the longitudinal axis of the screw piston and a plurality of radial bores 12 for distribution via the mixing elements 9.

The porous or permeable mixing elements 9 can be made of sintered metal or of another permeable material, such as e.g. Ceramic be formed. The bores 11, 12 are connected upstream of the input funnel 3 to a propellant supply device 10. For this purpose, a seal housing 18 with a housing core and screwable cover encloses the screw piston 1.

The seal housing 18 is mounted between a drive device (not shown) for the screw piston 1 and the plasticizing cylinder 2 and is secured against rotation. The seal housing 18 moves simultaneously with the axial movement of the screw piston 1. The axial stroke of the screw piston 1 corresponds, for example, to three times the diameter of the injection cylinder 2. The seal housing 18 has special rotary seals 19 and is centered on the screw piston with the aid of slide rings. Axial displacement of the seal housing 18 is prevented by mechanical clamping elements. Mechanical seals or radial shaft seals can be used as the rotary seals 19. One or more radial bores 20 connect the pressure chamber of the propellant supply device 10 to the axial bore 11 in the longitudinal axis of the screw piston 1.

After the blowing agent has been supplied over the surface of the mixing elements, promotional shear elements 21 distribute the polymer / blowing agent mixture. The blowing agent supply device 10 receives the blowing agent preferably via commercially available compressed gas cylinders. An electrically, pneumatically or hydraulically operated valve 22 connects the propellant supply, which may have been throttled with the aid of a pressure reducing valve, to the high-pressure seal housing 18 during the metering phase of the polymeric material. FIG. 2 shows an exemplary embodiment of the mixing elements. A rotationally symmetrical pin made of porous material 9 is in a threaded bore 14 perpendicular to Screw shaft 1 screwed. Alternatively, an interference fit or other clamping device can also be provided. Alternatively, spring-loaded projections could also be used, which engage in grooves in the screw piston. Such latching mechanisms can also allow disassembly of the mixing elements, which may be necessary for cleaning purposes. The mixing pin, like the bore in the screw piston 1, is offset and enables an axial sealing point 15 via the shoulder thus formed. With the aid of copper sealing disks or high-temperature-resistant O-ring seals 16, the mixing pin can thus be sealed against the screw piston and prevented an uncontrolled penetration of blowing agent into the plastic melt via the contact surface between the mixing pin and the screw piston. In the bottom of the screw thread there is a bore 12 which is radial to the axis of the screw piston and which meets the axial bore 11 in the screw piston and thus represents the connection to the propellant supply. In order to generate the most uniform possible outflow of the propellant over the surface of the mixing pin, this can optionally be provided with an axial bore 17. This ensures that the flow resistance through the permeable material is the same at all points on the surface. In addition to the cylindrical shape, the geometry of the mixing elements can also be conical. This has the advantage that the thermally induced inhomogeneities due to dissipation heating to the cylinder wall are reduced due to the decreasing end face. Furthermore, the mixing elements can be rectangular or diamond-shaped. 3 shows a development of the screw piston 1 in the area of the gassing points between the metering zone 6 and the shear zone 21 with the corresponding distribution of the mixing elements 9. A mixing element can consist of cylinders of different diameters, have a conical or frustoconical shape, have a diamond-shaped or rectangular cross-section, can be designed as a straight or oblique prism, or can represent a figure in the form of a serpentine or helical line.

LIST OF REFERENCE NUMBERS

1. Screw piston

2. Injection cylinder 3. Material hopper

4. Feed zone 5. Compression zone

6. Metering zone

7. Cylinder heating

8. Enlarged screw base 9. Mixing element

10. Propellant supply device

11. Hole

12. Radial bore

13. Gasing zone 14. Threaded hole

15. axial sealing point

16. O-ring seal

17. Axial bore

18. Seal housing 19. Rotary seal

20. Radial bore

21. shear zone

22.Valve

Claims

EXPECTATIONS

1. Device for producing foamed plastic molded parts in the injection molding process, preferably using a physical blowing agent, the injection molding machine containing at least one injection cylinder (2) which has at least one screw piston (1) which is at least through a feed zone (4), a compression zone (5 ) and includes a metering zone (6), the propellant preferably being introduced into an area adjoining the metering zone (6), characterized in that the propellant is only introduced in some, locally restricted areas, am

End (8) of the metering zone (6) the diameter of the screw piston is reduced and porous or permeable mixing elements (9) are provided.

2. Device according to claim 1, characterized in that a physical blowing agent is introduced into the polymer melt by the mixing elements (9).

3. Device according to claim 1, characterized in that the propellant supply device contains a bore (11).

4. The device according to claim 1, characterized in that the mixing elements (9) consist of sintered metal or ceramic.

5. The device according to claim 1, characterized in that at least one mixing element (9) is designed as a rotationally symmetrical pin.

6. The device according to claim 1, characterized in that each mixing element (9) is provided with a device for connection to the screw piston.

7. The device according to claim 6, characterized in that the device for connection to the screw piston comprises a threaded bore.

8. The device according to claim 1, characterized in that the mixing element (9) has at least one stepped region.

9. The device according to claim 8, characterized in that the remote area can accommodate a seal.

10. The device according to claim 9, characterized in that the seal consists of copper or a high temperature resistant O-ring.

11. The device according to claim 1, characterized in that a mixing element (9) consists of cylinders of different diameters.

12. The device according to claim 1, characterized in that a mixing element (9) has a conical or frustoconical shape.

13. The apparatus according to claim 1, characterized in that a mixing element (9) has a diamond-shaped or rectangular cross section.

14. The apparatus according to claim 1, characterized in that a mixing element (9) is designed as a straight or oblique prism.